Mitochondrial dysfunction is responsible for many human neurodegenerative diseases. The mechanism by which mitochondria senses and respond to stress is poorly understood. Recent results from our lab and others suggested that the mitochondrial serine protease Omi/HtrA2 is a sensor of mitochondrial stress. Inactivation of Omi/HtrA2 in mnd2 mice due to a point mutation in the protease domain or disruption of the Omi gene in Omi-/- knockout mice leads to neurodegeneration, muscle wasting and death by 40 days of age because of mitochondrial dysfunction. These finding suggests that Omi/HtrA2 has a vital role within the mitochondria that is required for maintenance of mitochondrial homeostasis at normal physiological conditions. Because Omi/HtrA2 shares extensive sequence and structural homology with the bacterial stress sensor DegS, we hypothesize that Omi/HtrA2, like DegS, is a stress sensor that becomes activated when its PDZ domain recognizes exposed PDZ-binding motifs in misfolded mitochondrial proteins. The activated Omi/HtrA2 then triggers a proteolytic cascade that leads to activation of specific stress response genes. The nature of the mitochondrial proteins and the types of unfolding stresses that trigger activation of Omi/HtrA2 are currently unknown. The nature of the mitochondrial Omi/HtrA2 substrates is also unknown. The mnd2 and Omi-/- mice are excellent model systems to study the role of Omi/HtrA2 in cell survival. To investigate the protective function and the signal transduction pathway of Omi/HtrA2, we propose in the first and second specific aims to carry out transgenic rescue experiments in Omi-/- and mnd2 mice to define the role of the PDZ and transmembrane domains of Omi in its protective function. We also propose to study phenotype of specific tissue expression of wildtype Omi in Omi-/- and mnd2 mice. In the remaining specific aims we propose to use biochemical, genetic, and proteomic approaches to identify and characterize the upstream mitochondrial regulators of the PDZ domain of Omi/HtrA2 and the critical mitochondrial substrates of Omi/HtrA2. We predict that Omi/HtrA2-mediated cleavage of a specific mitochondrial substrate could send a signal to the nucleus and activate transcription of Omi/HtrA2-specific stress responsive genes. We propose to identify these genes by microarray analysis. Understanding the molecular basis of stress sensing in the mitochondria and the regulation of the protease activity of Omi/HtrA2 is important for designing strategies for the treatment of mitochondrial-related neurodegenerative diseases. Mutations that reduce the activity of Omi/HtrA2 protease have recently been identified in patients with Parkinson's disease, suggesting a link between Omi/HtrA2 and neurodegeneration in some cases of Parkinson's disease and perhaps other human neurodegenerative diseases. Thus, learning about the function of Omi/HtrA2 under normal and disease conditions is a research objective of clear health interest.